Adjustable supporting surface



FIG. 1 is a perspective view of an adjustable supporting surface showing our new design;

FIG. 2 is a front elevational view;

FIG. 3 is a rear elevational view;

FIG. 4 is a left side elevational view;

FIG. 5 is a right side elevational view;

FIG. 6 is a top plan view; and,

FIG. 7 is a bottom plan view.

The broken lines are for environmental purposes only and form no part of the claimed design. 

The ornamental design for an adjustable supporting surface, as shown. 